The quality and quantity of red blood cells (RBC) in the blood stream is often degraded during periods of increased physical stress, resulting in anemia and enhanced risks of morbidity and mortality. Physical stresses that have been linked to the development of anemia include autoimmune diseases, major surgery, trauma, infectious diseases, cancer, critical illness, diabetes, liver diseases, kidney disease, heart failure, and parasitic diseases. Systemic inflammation is a characteristic common to all these situations as evidenced by the presence of increased levels of inflammatory cytokines in the circulation. Even in persons predisposed to anemia because of a hemoglobinopathy, for example sickle cell disease or thalassemia, inflammatory cytokine levels are frequently elevated and can exacerbate the disease symptoms, particularly during crisis episodes.
One consequence of elevated inflammatory cytokine levels is a reduction in the hepatic production of the enzyme lecithin:cholesterol acyltransferase (LCAT). Normally, LCAT is released into the plasma from the liver to facilitate plasma lipid turnover and maintain the balance of cholesterol and phospholipids in the blood and the tissues perfused by blood. Excess cholesterol is removed from tissues, such as arteries, and delivered to the liver for excretion in bile by a process known as reverse cholesterol transport (RCT). In the first step of RCT, cholesterol passes from tissue cells to high-density lipoproteins (HDL) in the circulation. In the second step, the enzyme LCAT enhances the cholesterol-carrying capacity of HDL by catalyzing the transesterification of a fatty acid from phosphatidylcholine (PC) (also known as lecithin), to cholesterol to form cholesteryl ester (CE). The CE product accumulates in the HDL interior until it is removed at HDL-receptors in the liver. The CE delivered to the liver by HDL is converted to cholesterol and bile acids that are excreted in the bile.
The health consequences of diminished plasma LCAT activity are most evident in persons with Familial LCAT Deficiency (FLD), a rare genetic disease in which plasma LCAT activity is absent. The absence of LCAT activity results in greatly diminished levels of plasma CE, reflected in decreased HDL and low-density lipoprotein, and in the accumulation of excess LCAT substrate in plasma. The major health consequences of FLD are reduced vision resulting from a diffuse build-up of lipid in the corneas, eventual kidney failure due to renal lipid accumulation (glomerulosclerosis), and hemolytic anemia.
Distortions in the plasma lipoprotein lipid compositions due to lipid metabolic disorders such as those resulting from low LCAT activity have been associated with changes in the lipid content of RBC. A shift in RBC lipids in response to plasma lipid changes can alter RBC performance and survival since these properties are dependent on cellular lipid content. The types of RBC lipid changes that can occur are evident in FLD subjects where the RBC are enriched in cholesterol and PC and diminished in sphingomyelin (SM) content. Evidence that these RBC lipid abnormalities depend on disturbances in plasma lipoprotein lipids as a result of LCAT deficiency was obtained in an experiment were a temporary normalization of RBC cholesterol content occurred following infusion of normal plasma into an FLD subject (Muryama et al. Am. J. Hematol. 16:129-137, 1984). This temporary normalization of the RBC lipids could be due to the replenishment of LCAT, HDL, apolipoprotein A-I or other plasma factors that are absent or greatly reduced in patients with FLD.
No link between anemia and LCAT activity is seen in less severe cases of diminished plasma LCAT activity. For example, patients with fish eye disease, a milder form of LCAT deficiency, exhibit less than 30% of normal plasma LCAT activity but have normal hemoglobin and hematocrit (Rousset et al. Curr. Opin. Endocrinol. Diabetes Obes. 16:163-171, 2009). Similarly, studies in subjects with liver disease found no correlation between lowered LCAT activity and anemia (L W Powell et al. (1975) Aust. N. Z. J. Med. 5:101-107), or between LCAT activity and RAC lipid abnormalities. (R A Cooper et al. (1972) J. Clin. Invest. 51:3182-3192).
Although there is evidence of deleterious lipid alterations in RBC in persons under physical stress that are similar to those detected in FLD patients, there is no apparent relationship between LCAT and RAC level or lipids. Examples of anomalous RBC lipid composition include reports of increased PC/SM ratio in RBC from persons with liver disease and in persons with dyslipidemia due to lipoprotein lipase deficiency or Tangier Disease. We (FIG. 1) and others have also found an increase in the PC/SM ratio in RAC from sickle disease patients who are not in crisis. Furthermore, there are reports of cholesterol enrichment in RBC from persons with diabetes, heart disease (including acute coronary syndromes), hypercholesterolemia, sickle cell anemia, and in persons after space flight.
The consequences of modified RAC lipid composition are not fully known but in the case of elevated RBC cholesterol there is evidence that activities of membrane proteins become abnormal. Cholesterol-enriched RBC from liver disease patients exhibit reduced activities of Mg++-ATPase and acetylcholine esterase. Cholesterol enrichment has been linked to enhanced transfer of phosphatidylserine from the inner to the extracellular membrane surface, which is a signal for enhanced clearance of RAC by the reticulo-endothelial system. Increased RBC cholesterol can reduce RBC deformability and induce abnormal RBC morphologies, both of which can impair RBC transit through the capillaries. Transmembrane gas exchange, an essential RBC function, is also impacted by cholesterol elevation.
The current evidence suggests abnormal RAC lipid compositions can have a deleterious effect on red blood cell function and therefore there is a need for methods to normalize RAC lipid composition and methods to treat red blood cell dysfunction.